PI3 kinase delta governs regulatory T-cell biology and thus confers protection against atherosclerosis progression in mice

Abstract Introduction and Purpose Atherosclerosis is a chronic inflammatory disease of arteries, critically involving innate and adaptive immune cells like macrophages as well as T and B lymphocytes. Macrophages are major drivers of disease through the ingestion of lipoproteins, foam cell formation, and secretion of inflammatory mediators. Although macrophages outnumber other leukocytes in atherosclerotic plaques, T and B cells can shape the course of disease by promoting or mitigating inflammatory responses. The phosphoinositide 3-kinase delta (PI3Kd) exerts a key role in the regulation of immune responses including activation, proliferation, differentiation, and effector function of lymphocytes. Therefore, PI3Kd may represent a promising target for the modulation of inflammatory diseases. Consequently, we aimed to analyse the role of PI3Kd during atherogenesis. Methods and Results To investigate the role of haematopoietic PI3Kd in atherosclerosis, bone marrow from PI3Kd-/- or PI3Kd+/+ mice was transplanted into Ldlr-/- mice. After a 6-weeks-challenge by high fat diet, PI3Kd-/- recipient Ldlr-/- mice displayed profoundly impaired CD4+ and CD8+ T-cell numbers, CD4+ T-cell activation, CD4+ effector T-cell differentiation, and proatherogenic CD4+ T-helper (Th) 1 responses in para-aortic lymph nodes and spleen compared with PI3Kd+/+ transplanted controls. Surprisingly, the net effect of PI3Kd deficiency was a substantial increase of aortic inflammation and atherosclerosis in Ldlr-/- mice. Whereas plaque content and functions of macrophages including foam cell formation, efferocytosis, and cytokine secretion remained largely unaffected, haematopoietic PI3Kd ablation strongly reduced mature B cells and serum immunoglobulins in Ldlr-/- mice. Importantly, PI3Kd deficiency severely impaired numbers, proliferation, immunosuppressive functions, and stability of regulatory CD4+ T cells (Tregs). Consequently, adoptive transfer of PI3Kd+/+ Tregs fully restrained the atherosclerotic plaque burden in PI3Kd-/- transplanted Ldlr-/- mice without affecting B-cell numbers and serum immunoglobulins, whereas transferred PI3Kd-/- Tregs failed to relieve atherosclerosis progression. Conclusions Here, we demonstrate that PI3Kd plays a crucial role in Tregs, Th1 cells, and B cells during atherogenesis. Lack of PI3Kd signalling specifically in atheroprotective Treg responses outplays its impact on B-cell and proatherogenic Th1 responses, thus leading to aggravated atherosclerosis. Hence, PI3Kd is a key regulator of Treg biology and thereby protects against atherosclerosis, suggesting that fine-tuning PI3Kd signalling may represent a promising target for Treg-directed therapy.